Transformer oils are formulated so that they may meet or exceed certain specific, performance conditions exemplified by ASTM D3487 and CSA C-50 requirements. These conditions include a minimum pour point, a maximum kinematic viscosity and enumerated limits on interfacial tension, gassing tendency and levels of acid number and sludge produced at 24 and 164 hours in the ASTM D2440 oxidation test. In the past only transformer oils produced from extracted-hydrofinished naphthenic distillates met or exceeded the demanded performance characteristics.
Attempts have been made to produce transformer oils from feed stocks other than naphthenic oils.
U.S. Pat. No. 4,124,489 teaches a process for producing transformer oil by double solvent extracting a raw, untreated, light distillate fraction from a waxy crude oil to produce a second, wax containing extract oil. This second extract oil is hydrotreated to mildly crack it, reduce the sulfur content and improve the viscosity, oxidation and color stability. This hydrotreated oil is then distilled to produce a transformer oil feedstock of relatively low wax content as a heart cut fraction having a 5 to 95 LV% boiling range between about 595.degree. to 750.degree. F. The transformer oil feedstock may then be dewaxed using any well known method such as solvent or catalytic dewaxing to obtain a low pour point transformer oil.
U.S. Pat. No. 4,018,666 teaches a process for producing a very low pour point transformer oil by a process wherein a narrow cut distillate of a paraffinic crude from conventional crude oil atmospheric or vacuum towers is first solvent extracted to remove aromatics and polar components, followed by immiscible solvent dewaxing whereby two liquid and one solid phases form a wax-containing slurry which is filtered to produce a wax cake which contains a high viscosity index oil and a filtrate which contains a very low pour point transformer oil.
U.S. Pat. No. 4,062,791 teaches an electrical insulating oil having excellent oxidation stability, thermal stability, corona resistance, corrosion resistance and a low pour point. This oil consists essentially of a blend of a solvent extracted, hydrofined and dewaxed oil derived from a paraffin or mixed base crude oil, a solid adsorbent treated oil prepared from a lubricating oil fraction of a mineral oil, at least one arylalkane such as alkylbenzene and, if desired, an essentially amorphous ethylene propylene copolymer. The oil has a sulfur content of not more than 0.35 wt%.
U.S. Pat. No. 4,069,165 teaches an electrical insulating oil consisting essentially of a mineral oil containing not more than 0.35 wt% sulfur prepared by solvent extracting, hydrofining, and dewaxing a distillate containing at least 80 wt% of a fraction boiling at 230.degree. to 430.degree. C. at atmospheric pressure, the distillate being obtained by the distillation of paraffins or mixed base crude oils, at least one arylalkane and if desired a hydrocarbon derived pour point depressant.
U.S. Pat. No. 4,664,775 teaches a method for manufacturing low pour point petroleum products from paraffin base oils using a zeolite for the catalytic dewaxing step.
U.S. Pat. No. 3,684,695 teaches a process for hydrocracking an oil to produce high viscosity index lubricating oils. A high boiling hydrocarbon oil, such as a deasphalted residual oil is hydrocracked over a catalyst, a liquid product boiling in the 350.degree. to 550.degree. range is recovered and dewaxed.
U.S. Pat. No. 3,365,390 teaches a process for producing lubricating oils. The lube oil is produced by hydrocracking a heavy oil feed, separating hydrocracked wax, hydroisomerizing the hydrocracked wax, dewaxing the isomerate by itself or in admixture with the hydrocracked lube oil portion. An additional hydrogenation step may precede and/or follow the wax isomerization step.
GB 1,440,230 teaches a process for preparing lube oils. The process involves catalytic hydrocracking a high boiling mineral oil fraction (e.g. a vacuum distillate boiling at between 350.degree. and 500.degree. C. or a deasphalted residual oil). After hydrocracking the hydrocarbons boiling below the range between 350.degree. and 400.degree. C. are removed by distillation and the higher boiling residua is dewaxed yielding a high VI lube oil. The wax is hydroisomerized to increase the yield and improve the VI of the final oil product.
GB 1,493,928 teaches a process for the conversion of hydrocarbons. Lubricating oils are produced by the catalytic hydrocracking of heavy hydrocarbons, said heavy hydrocarbons consisting at least partially of one or more foots oils and, optionally, of other heavy fractions selected from waxy lube oil fractions obtained during the distillation under reduced pressure of atmospheric distillation residues of waxy crudes, slack waxes separated from the aforesaid waxy lube oils or slack waxes separated from waxy lube oils obtained by hydrocracking.